Microminiature resonant sensors fabricated in semiconductor materials have gained increased use in the precision pressure measurement field. These sensors have high stability, high sensitivity and low temperature coefficients. Their stability and accuracy are less dependent on electronic signal processing circuitry than are comparable piezoresistive sensors and capacitive sensors. Their construction, however, is generally more complex than piezoresistive pressure sensors.
Resonant sensors utilize a vibrating micromechanical body or resonator and provide a frequency as output data. The frequency output depends upon a stress modifying the natural resonant vibrational frequency of the resonator. The behavior of a resonator, for example, a vibrating beam, is somewhat analogous to that of a stretched string. An increase in tension in the string causes an increase in the resonant frequency. In a resonant sensor, a load applied to the structure results in a strain in the resonator causing a shift of the resonator's resonant frequency. The frequency output of the resonator provides a measure of the magnitude of the mechanical load applied to the sensor structure. Physical parameters such as an acceleration, pressure, force, and temperature can be measured as a consequence of the frequency shift. Analog and/or digital electronics interface with the varying frequency output, with high resolution achievable.
Sensors and gauges are often ascribed gauge factors. The gauge factor of a resonant strain gauge or resonator is basically the rate of change of frequency per unit strain and is a measure of the sensitivity of the gauge. Gauge factors of resonant strain gauges are typically orders of magnitude higher than gauge factors for piezoresistive strain gauges, e.g., 3,000 compared to 30. Resonant sensors are thus very attractive for measuring small mechanical loads with a high resolution. Unlike piezoresistive gauges, the gauge factor of resonant gauges is not dependent on the impurity concentrations of the materials of the gauge, but rather dependent on resonator dimensions. For a beam resonator, the gauge factor depends on the aspect ratio, L/h, where L is the length of the beam and h is the thickness of the beam, and on the residual strain.
Microminiature resonant sensors are generally fabricated in semiconductor materials, primarily silicon-based materials, using standard processes which have been developed in the semiconductor art such as thin film deposition, etching, doping and lithography. These processes permit the formation of very small, complex three-dimensional structures. Such structures can be made reproducibly and in large numbers.
Several structures for the miniature silicon resonators have been described, including resonant diaphragms, cantilevers, and beams of various shapes. The latter are typically suspended over a silicon diaphragm of uniform thickness. Because the resonator is coupled to a diaphragm, displacements of the diaphragm due to an applied load distort the resonator, modifying the resonant frequency of the resonator. See, e.g., K. Ikeda et al., British Patent Application GB 2180691A; S. C. Greenwood, British Patent 1,596,982; Thornton et al., "Novel Optically Excited Resonant Pressure Sensor, Electron Lett., Vol. 24 (1988), pp. 573-574; Smits et al., "Pressure Dependence of Resonant Diaphragm Pressure Sensor," Third International Conference on Solid-State Sensors and Actuators (1985), pp. 93-96; J. C. Greenwood, "Etched Silicon Vibrating Sensor", J. Physics E. Sci. Instrs., Vol. 17 (1984), pp. 650-652.
Various placements for resonant beams with respect to a diaphragm have been described. Typically, the resonator is located at the center of the diaphragm. See, e.g., Greenwood, J. Physics E. Sci. Instrs., Vol. 17 (1984) pp. 650-652. But Ikeda et al., "Three-Dimensional Micromachinery of Silicon Resonant Strain Gauge," Technical Digest of the 7th Sensor Symposium (1988), pp. 193-196, describe a sensor with one resonator at the center and another at the edge of the diaphragm.
In some sensors the resonator is hermetically sealed in an evacuated cap or enclosure (see, Ikeda, Technical Digest of the 7th Sensor Symposium (1988) pp. 55-58). The resonator in this architecture is separated from its surroundings, eliminating effects such as the dependency of frequency on the specific mass of the surrounding medium. The air damping of the resonator will also be reduced resulting in a high quality factor. A quality factor is a measure of the energy loss of a resonator. Quality factors are always finite, i.e., energy must be supplied to the resonator or it will stop vibrating. High quality factors allow the fabrication of high resolution devices, i.e., small changes in the load can be detected by measuring the (small) shifts in the frequency.
A sealing cap, however, may become an important design parameter of the mechanical structure of the sensor. Since the cap must endure the same base pressure as the sensor, e.g., one atmosphere, it must have a certain minimum thickness to avoid collapse. If the thickness of the cap is similar to the thickness of the diaphragm, the cap partly determines the stress distributed in the diaphragm. For a uniform diaphragm sensor, the cap will, for example, cause the neutral plane of the diaphragm to rise, i.e., come closer to the resonator. As a result, this will reduce the induced bending stresses, thus lowering sensitivity. Furthermore, the cap strongly impacts the position of the inflection points of the diaphragm, i.e., points of zero bending stresses. The position of these inflection points is not very predictable, which further complicates placement of the resonators.
For operation, the resonators require excitation into vibrational motion and detection of this motion. An exciter exerts forces and moments on the resonator which bend and elongate or contract the resonator. If the frequency of excitation is the same as the mechanical resonant frequency, the resonator vibrates with a large amplitude. The detector picks up the vibratory motion and its output signal will show a resonance peak. Excitation and detection elements must thus be included in the sensor structure. Excitation methods include thermal, optical, electrostatic, piezoelectric and magnetic. Detection methods include piezoelectric, piezoresistive, capacitive and optical. Examples of excitation/detection pairs described in the prior art include: thermal excitation/piezoresistive detection; electrostatic excitation/capacitive detection; Lorentz force excitation/magnetic flux detection; piezoelectric excitation/piezoelectric detection, e.g., using ZnO; and optical excitation/optical detection.
Thermal excitation and piezoresistive detection are very attractive from a technological point of view. See, for example, T. S. J. Lammerink et al., "Integrated Thermally Excited Resonant Diaphragm Pressure Sensor", Proceedings of the Third International Conference on Solid-State Sensors and Actuators, Transducers '85 (1985), pp. 97-100. Fabrication is relatively easy. However, heat dissipation is an inherent problem. Dissipated heat will cause thermally induced stresses in a sensor which will affect the resonant frequency. Highly accurate devices impose stringent requirements on the control of the heat dissipation and heat flow.
The technology for fabricating the elements required in electrostatic excitation/capacitive detection is also relatively simple. Capacitive detection, however, is known to be a very insensitive method. This detection method generally requires on-chip circuitry which complicates the sensor fabrication process. Also, suppression of parasitic capacitances requires special attention.
The Lorentz force excitation/magnetic flux detection method, in which heat dissipation is also an inherent problem, requires a permanent magnet proximate to the resonator. See, for example, Ikeda et al., "Silicon Pressure Sensor With Resonant Strain Gauges Built Into Diaphragm," Tech. Digest of the 7th Sensor Symposium (1988), pp. 55-58. Thus, packaging of a sensor using this technology is more complicated than other methods.
Optical excitation and detection is difficult to implement if the resonator is housed in a cap. See, for example, D. Uttamchandani et al., "Optically Excited Resonant Beam Pressure Sensor", Elect. Letters, Vol. 23 (1987), pp. 1333-1334 and Thornton et al., "Novel Optically Excited Resonant Pressure Sensor," Elect. Letters, Vol. 24 (1988), pp. 573-574. The cap material must be transparent to the light used for excitation. Also, optical detection requires a complicated optical interferometer set up.
The use of piezoelectric thin films, in particular, zinc oxide films, offers a very efficient way for both excitation and detection of the motion. The sensor fabrication process, however, is complicated because zinc oxide is not a very integrated circuit-compatible material. Additionally, the residual stress of the film must be controlled.
Despite attempts to provide accurate, simple microminiature resonant sensors, prior art devices suffer, among other things, from use of bulky excitation/detection methods which result in awkward packaging, and nonlinearity between frequency output and applied force due to temperature effects, placement of resonators and use of hermetic sealing caps. The present invention provides a resonant sensor in which parameters involving material properties of the resonators, temperature effects, and long term drift are essentially not present in the final output frequency signal. The sensor provides a resonator built in or integral to the diaphragm, allows for very accurate control over diaphragm thickness, exhibits symmetric strain under front and back pressures, and virtually eliminates the resonator sealing cap being a determinant of mechanical behavior of the diaphragm.